Medical devices may be coated to provide localized delivery of therapeutic agents to target locations within the body. The therapeutic agents generally may treat localized disease (e.g., heart disease) or occluded body lumens or may mitigate undesirably side effects or costs of systemic drug administration. Localized drug delivery may be achieved, for example, by coating endoluminal devices such as balloon catheters, stents and the like with the therapeutic agent to be locally delivered.
Endoluminal devices may be configured to bring the coating into therapeutically effective contact with the wall of a body vessel. For instance the endoluminal devices may be a radially expandable tubular stent formed by a plurality of interconnected members defining open cells extending between an external (abluminal) surface and an internal (luminal) surface. The therapeutic agent may be applied to the abluminal surface of the endoluminal device for delivery to a treatment site within a body vessel. The luminal surface defines a tubular lumen extending axially from the proximal end to the distal end of the endoluminal device.
When a coating containing the therapeutic agent is applied to the abluminal surface of the endoluminal device, it is desired to provide a uniform coating in order to minimize the coating of the luminal surface. In addition, the therapeutic agent is preferably localized on the interconnected members (e.g., struts and bends) of the endoluminal device, rather than being present within the open cells between these members. Upon radial expansion of the endoluminal device, the distance between adjacent members typically increases and the area enclosed by the open cells between these members typically increases. As such, therapeutic agent coated over, or bridging, such open cells may fall through the cells, into the lumen and be undesirably washed away by the blood from the point of treatment without contacting the wall of the body vessel.
Such coated device structures are commonly deployed within a body vessel to maintain patency of a stenosis, and the therapeutic agent may be selected to mitigate or prevent restenosis of the body vessel after dilation. For example, the endoluminal device may be delivered endovascularly using a catheter delivery system by expanding the endoluminal device from a radially compressed delivery configuration within a portion of the catheter to a radially expanded configuration within the body vessel. The endoluminal device delivery may be performed as part of a procedure to dilate a blood vessel with the catheter balloon, such as percutaneous transcoronary angioplasty (PCTA). The endoluminal device may be radially expanded by a balloon attached to the catheter or may be formed of a material that radially self-expands when released from the catheter.
Coatings have been applied to medical devices by processes such as dipping, spraying, vapor deposition, plasma polymerization, and electrodeposition. Although these processes have been used to produce satisfactory coatings, they have numerous, associated potential drawbacks. For example, it may be difficult to achieve coatings of uniform thicknesses, both on individual parts and on batches of parts. Also, these coating processes may require that the coated part be held during coating, which may result in defects such as bare spots where the part was held and which may thus require subsequent coating steps. Further, many conventional processes require multiple coating steps or stages for the application of a second coating material, or to allow for drying between coating steps or after the final coating step.
One method of coating endoluminal devices involves mounting an endoluminal device on a mandrel, spraying a solution of the therapeutic agent, and applying a suction force within the mandrel. The solution includes a volatile solvent and is sprayed onto the abluminal surface of the mounted endoluminal device. The solvent is allowed to evaporate, leaving the abluminal surface coated with the therapeutic agent. Optionally, a polymer may be dissolved in the solution with the therapeutic agent and solvent, or applied with the solvent to form a separate coating layer from the therapeutic agent.
One difficulty with the above-described method of coating the endoluminal device is the potential for coating defects, and inadvertent application of a coating to the luminal surface during coating of the abluminal surface. While some coating defects can be minimized by adjusting the coating parameters, other defects occur due to the nature of the interface between the endoluminal device and the mandrel on which the endoluminal device is supported during the coating process. Typically, a high degree of surface contact between the endoluminal device and the supporting apparatus can provide regions in which the liquid composition can flow, wick, and collect as the composition is applied. As the solvent evaporates, the excess composition hardens to form excess coating at and around the contact points between the tubular medical device and the supporting apparatus, also referred to as “webbing” of the coating.
Upon the removal of the coated endoluminal device from the supporting apparatus, the excess webbed coating may stick to the apparatus, thereby removing some of the needed coating from the endoluminal device and leaving bare areas. Alternatively, the excess coating may stick to the endoluminal device, thereby leaving excess coating as agglomerations or pools on the struts or webbing between the struts. During implantation of the coated endoluminal device, excess therapeutic agent deposited within the openings in the frame of the endoluminal device may be dislodged upon radial expansion of the coated endoluminal device and fall through the openings into the lumen of the coated device.
Coating mandrels may be coupled to vacuum sources to remove excess coating material from a medical device during the coating process. For example, U.S. Pat. No. 6,818,063 to Kerrigan, filed Sep. 24, 2002, describes a stent mandrel fixture for supporting a stent during the application of a coating substance that includes a hollow perforated central mandrel in fluid communication with a vacuum device. A fluid coating composition applied to a stent mounted around the mandrel passes through openings in the stent, into the perforations in the mandrel and through a bore formed within the mandrel that is in fluid flow communication with the vacuum device. However, the bore is not structured to facilitate the rapid removal of coating solution fluid through the perforations. For example, eddying currents within the bore may cause uneven rates of suction through the perforations, permitting pooling of coating solution on the surface of the mandrel between openings in the stent. This can lead to webbing of the coating, clogged perforations and/or coating deposition on the abluminal surface of the stent. Thus, there remains a need for coating methods and structures useful to minimize the coating contact with (and deposition of) a coating on the luminal surface of the endoluminal device. There is also a need to minimize webbing of the coating in openings of the coated medical device or agglomeration of coating material on the struts.